Field of the Invention
The invention relates to a piezoelectrically operable valve with at least one valve channel, which can be closed by means of a sealing element held on a piezoelectric bending transducer.
DE 36 08 550 A1 discloses a piezoelectrically operable valve, into the valve housing of which at least three valve channels open out to form a three-way valve. The valve, which can be used as a pneumatic valve or hydraulic valve, has within its valve housing at least one piezoelectric bending transducer, the connecting end of which, for connecting an electric voltage source, is fixed in a narrow side of the housing. The free end of the bending transducer, protruding into the interior space of the housing, carries on its side facing the respective valve channel a sealing element for closing this valve channel.
In order, in an initial state or state of rest with an outflow channel open, to close an inflow channel lying opposite said outflow channel, the corresponding bending transducer is mechanically prestressed and/or pressed against the valve seat of the inflow channel by means of spring force. In this state, the opposite outflow channel is opened by a sealing element which is held by the same or a further bending transducer, that is fixed in the housing on the connection side, being situated opposite the valve seat of the outflow channel at a distance from it.
In the operating state, the connecting end of the bending transducer or of each bending transducer is connected to a voltage source, the free end of the bending transducer being deflected in the direction of the respective valve channel or in the opposite direction, depending on the polarity. As a result, the sealing element moved along with the free end of the bending transducer closes the outflow channel while the inflow channel is at the same time opened. As a result, a medium flowing into the valve housing via the inflow channel is carried away via a working channel, whereas in the state of rest, with the inflow channel closed, a medium flowing in via the working channel flows out of the valve housing via the outflow channel.
In the case of this known piezoelectrically operated valve there is the problem, in particular under extreme operating conditions, of inadequate tightness of the valve channel closed according to the operating state. Consequently, when a medium is passed via the valve at a high pressure and/or high flow rate, undesirably high leakage rates may occur as a result of inadequate sealing, in particular of the valve channel that is open in the state of rest and is to be closed by means of the excited bending transducer.
A piezoelectrically operated valve known from U.S. Pat. No. 5,630,440 has a bending transducer that is clamped in the housing at both ends and is consequently fixed by both its ends. As a result, a comparatively high restoring force or working force is admittedly achieved in comparison with fixing of just one end of the bending transducer. However, the fixing of the bending transducer at both ends has the considerable disadvantage of an only small, and therefore generally inadequate, deflection displacement or bending excursion.
The invention is therefore based on the object of specifying a piezoelectric valve, in particular a pneumatic valve, in which the valve channel to be blocked is reliably closed, while the disadvantages mentioned are avoided.
This object is achieved according to the invention by the features of claim 1. For this purpose, the second end of the bending transducer, lying opposite the fixed first end, is guided in a longitudinal groove provided in the valve housing. This guidance, and the mobility of the bending transducer in the excited state achieved as a result, permits a longitudinal movement of the bending transducer, while a lateral movement of the free end is prevented by the groove side walls forming stops on both sides for the free end when the bending transducer is excited.
In this case, the fixed end is expediently configured as a connecting end which can be connected to an electric voltage source for valve operation.
The invention is in this respect based on the idea that a reliable closing of the valve channel to be blocked, and consequently deactivated in each case, can be achieved even under extreme operating conditions, in particular in the case of a high pressure and a high flow rate of a medium flow to be controlled, by increasing the force, with at the same time a deflection displacement or bending excursion that is as great as possible. This increased pressing force with which the sealing element is pressed against the valve seat of the corresponding valve channel by means of the respective bending transducer can be produced by the bending transducer itself, with at the same time an adequate deflection displacement, if said bending transducer is held at both ends within the valve housing and at the same time is fixed only at one end, while the other end is merely supported in the manner of an abutment. As a result, an improvement in the operating behavior of the bending transducer and an increase overall in the operating reliability of the valve are achieved.
The invention is based here on the finding that, with the given connected voltage or operating voltage and accordingly with a constant bending moment of the piezoelectric transducer, the force produced by the latter increases with a decreasing lever arm. If the bending transducer is consequently held at both ends, the pressing force exerted by the sealing element on the valve seat of the corresponding valve channel as a result of a bending deflection of the bending transducer is distinctly greater in comparison with a bending transducer that is clamped at one end and freely movable at the free end. If in this case one of the ends of the bending transducer is guided in an axially movable manner, the deflection displacement is at the same time greater than in the case of a bending transducer fixed at both ends. Therefore, it is expedient to arrange the sealing element in the central region of the bending transducer, preferably with at least approximately the same distance both from the fixed end and from the movably guided end.
In order to achieve reliable closing of a valve channel already when it is in the initial state or state of rest in a particular simple and effective way, in an advantageous development the bending transducer is concavely bent in the direction of this valve channel in the de-energized state. The bending profile of the bending transducer in the de-energized or excitation-free state of rest is expediently achieved by the bending transducer itself being prestressed in the manner of a leaf spring. Such prestressing is in turn expediently achieved by a suitable production process. In this respect, a laminated bending transducer with a layer structure having at least one electrically conductive backing layer and a piezoceramic layer is taken as a basis and, by heating and subsequent cooling, leads to layers prestressed to varying degrees (prestressed layers) as a result of material-dependently different contraction properties. The different prestressing of the layers in turn leads to a bending deflection of the flat bending transducer about a bending axis running transversely with respect to its longitudinal sides and lying parallel to its narrow sides.
In a particularly preferred embodiment, two bending transducers are arranged running essentially parallel to each other within the valve housing. Their respective connecting end, which can be connected to an electric voltage source, for valve operation is in turn expediently the end that is held fixedly on the housing, while the respective free end is in turn held movably in the longitudinal direction. With respect to a central axis of the housing, the two bending transducers are convexly curved in the direction of the valve channel respectively assigned to them. In the de-energized state, a first valve channel is in this case closed by means of a sealing element, carried by a first bending transducer, while an opposite second valve channel is open. Lying opposite this second valve channel, at a distance from it, there is then a sealing element carried by the second bending transducer.
By applying the operating voltage with appropriate polarity, both bending transducers are excited, so that the first valve channel is opened and the second valve channel is closed. A medium flowing into the valve housing via an additional inflow channel is consequently led out of the valve housing via the second channel in the state of rest and via the first channel in the operating state. Depending on the mode of operation, a medium flowing in via the open second valve channel can flow away out of the valve housing via the inflow channel.
When a single bending transducer is used, in the de-energized state it is concavely bent in the direction of a first valve channel, a second valve channel, lying opposite said first valve channel, extending within the valve housing to within the effective proximity of the concave side of the bending transducer. In particular in this embodiment with only a single bending transducer, the latter carries a sealing element that is effective on both sides of its central region.